A standard military and/or commercial, panel mounted, indicating fuse holder is disclosed in U.S. Pat. No. 2,989,610 to W. C. Linton dated June 20, 1961, wherein a standard cartridge type fuse is housed and provided with a lamp so that when the fuse blows, the lamp will glow to indicate the inoperative condition of the fuse. In non-indicating fuse holders, the lamp is omitted.
In order to meet the stringent EMI/RFI emission control standards of the various electrical equipment in which the standard fuse holders are employed, it has become necessary to provide the fuse holders with an EMI/RFI shield. While the concept of providing an indicating fuse holder with an EMI shield has been suggested in U.S. Pat. No. 3,924,216 to Martin dated Dec. 2, 1975, the shield cap assembly of the present invention is an improvement thereon, and comprises, essentially, a metal cap threadably connected to a nut which also secures the fuse holder housing to the panel in which it is mounted. A rubber O-ring is mounted within the nut and is engaged by the inner end of the cap when threaded onto the nut, and an electrically conductive O-ring is mounted in the nut and is engaged by the face of the panel on which the fuse holder housing is mounted. The closed end of the cap is provided with a plurality of small apertures for viewing the light on an indicating-type fuse holder, and a plastic tether is provided to prevent loss of the cap when the cap is removed from the operative position for changing a fuse.
By the construction and arrangement of the shield cap assembly of the present invention, the cap can be added, or field retrofitted, to standard military and/or commercial fuse holders installed in equipment, while in use, without removal, major disassembly, or wiring change. The cap, associated nut and electrically conductive O-ring provide a metallic capsule completely enclosing the fuse holder and panel aperture in which the assembly is mounted. The aperture pattern and size in the closed end of the cap are configured and arranged to provide optimum light transmission during blown fuse indication with minimum EMI radiation.
Other advantages provided by the shield cap assembly of the present invention will become apparent to those skilled in the art during the course of the following detailed description.